Improve pre conditions secure boot

Improve the wording of the pre conditions section in the secure boot test
to instruct the flashing to the correct u-boot image based on the release
under test.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

test-cases/secure-boot-imx6.yaml

@@ -14,9 +14,9 @@ metadata:
 
   pre-conditions:
     - "Requires the DUT to be flashed with a signed U-Boot built with HAB and FIT image support
-      as provided since version 2019.01+dfsg-7co6. U-Boot could be installed with
-      [public U-Boot installer v2021.0](https://images.apertis.org/release/v2021/v2021.0/installer/mx6qsabrelite-uboot/)
-      or any newer version."
+      as provided since version 2019.01+dfsg-7co6. U-Boot could be installed from folder
+      installer/mx6qsabrelite-uboot in https://images.apertis.org/release/ corresponding to the
+      release to test"
     - "Requires a DUT with fused SRK hash (it's a irreversible operation! Please refer to the documentation).
       Or use additional step for 'emulation' if fusing is not possible. Skip it if the board has fused SRK."
     - 'It is expected that DUT is working in "open" HAB mode'